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November 25, 2009

Cancer Detectors

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Quantum dots for biomedicine

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More people are surviving cancer

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After any cancer surgery, your biggest question is "Did the surgeons get it all?" But looking for sites where cancer might have spread is hard to do, and can be tough on the patient. As this ScienCentral News video reports, there may be a way to light up those sites— a big help to both surgeon and patient.

X-Ray Vision

Often, the first step doctors take to determine whether cancer has spread from a tumor to other parts of the body is to check a patient's lymph nodes, small bean-shaped structures that are part of the immune system. The body's lymphatic system drains fluid and cells to lymph nodes, which contain cancer-fighting white blood cells. In particular, surgeons look for the sentinel lymph node, the first lymph node to which a cancerous tumor drains.

Removing only the sentinel lymph node and examining it for signs of cancer is less invasive than traditional surgeries. "The surgeon attempts to find the one lymph node from many in the area to which the tumor would drain," explains John Frangioni, assistant professor of medicine at Harvard Medical School, and assistant professor of radiology at Beth Israel Deaconess Medical Center. "Now this is important because when this lymph node is found, it can spare the patient from unnecessary surgery of the nodes."

However, Frangioni says, current methods aren't sensitive enough to allow surgeons to easily locate and remove that one tiny sentinel lymph node. To be safe, they often have to remove several lymph nodes, and that's tough on a patient. But what if that one lymph node that could be cancerous lit up during surgery— so brightly that a surgeon could see it through the patient's skin before making an incision?

Enter Frangioni's research partner, Massachusetts Institute of Technology chemist Moungi Bawendi. Bawendi works with quantum dots, tiny bits of semiconducting material that can emit very bright, invisible light, depending on their size. "Semiconductors are the kinds of material that run your computers, and quantum dots are pieces that contain maybe a few thousand atoms of these kinds of materials," says Bawendi. "Because quantum dots are so small, they have properties that are very different from the properties of the semiconductors that would be in your computer. They have optical properties that depend strongly on the size of the dot. By changing the number of atoms, and the types of atoms inside a dot, we can tune the color that a dot can generate."

quantum dots
Exposed to ultraviolet light, tiny quantum dots can emit bright intense colors whose strength and hue depend on the dots' size.
image: Quantum Dot Corporation
Bawendi designed dots only a few atoms in diameter that could be used in the body. His dots are small enough to travel through lymphatic fluid, but large enough to get caught in lymph nodes, where they collect and glow a bright neon green on the computer screen, making them much easier for a surgeon to find.

At Beth Israel Deaconess Medical Center, Bawendi and Frangioni have been collaborating with a team of surgeons who have been using quantum dots to find lymph nodes in the colons of pigs. Although the light that quantum dots emit is invisible, Frangioni has designed a system of cameras that makes the dots' light visible for the surgical team. They inject the quantum dots, and then watch on a computer monitor as the dots travel to and collect in a single lymph node. Frangioni explains that in only a minute or two, even through thick tissue, "the surgeon can watch the migration and flow of the dots into the lymph system, the flow to the sentinel node, and the accumulation in the sentinel node, all in real time." Once a surgeon has excised the sentinel node, "the next step in the process is to have the pathologist look very carefully under the microscope for any tumor cells that may be hiding in that lymph node."

Frangioni and Bawendi are working on using their system to detect cancer's spread in any part of the body, as well as to detect tumors. This research was published in the January, 2004 issue of the journal Nature Biotechnology and was funded by the National Science Foundation (NSF), the Office of Naval Research (ONR), the Stewart Trust, the Department of Energy (DOE), and the National Institutes of Health (NIH).

       email to a friend by Karen Lurie

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